The subject matter relates to a temperature device for use in the estimation of deep tissue temperature (DTT) as an indication of the core body temperature of humans or animals. More particularly, the subject matter relates to constructions of zero-heat-flux DTT measurement devices.
Deep tissue temperature measurement is an estimate of the temperature of organs that occupy cavities of human and animal bodies (core body temperature). DTT measurement is desirable for many reasons. For example, maintenance of core body temperature in a normothermic range during the perioperative cycle has been shown to reduce the incidence of surgical site infection; and so it is beneficial to monitor a patient's core body temperature before, during, and after surgery. Of course noninvasive measurement is highly desirable, for the safety and the comfort of a patient, and for the convenience of the clinician. Thus, it is most advantageous to obtain a noninvasive DTT measurement by way of a device placed on the skin.
Noninvasive measurement of DTT by means of a zero-heat-flux device was described by Fox and Solman in 1971 (Fox R H, Solman A J. A new technique for monitoring the deep body temperature in man from the intact skin surface. J. Physiol. January 1971:212(2): pp 8-10). The Fox/Solman system, illustrated in FIG. 1, estimates core body temperature using a temperature measurement device 10 with a controlled heater of essentially planar construction that stops or blocks heat flow through a portion of the skin. Because the measurement depends on the absence of heat flux through the skin area where measurement takes place, the technique is referred to as a “zero heat flux” (ZHF) measurement. Togawa improved the Fox/Solman technique with a DTT measurement device structure that accounted for multidimensional heat flow in tissue. (Togawa T. Non-Invasive Deep Body Temperature Measurement. In: Rolfe P (ed) Non-Invasive Physiological Measurements. Vol. 1. 1979. Academic Press, London, pp. 261-277). The Togawa device, illustrated in FIG. 2, encloses Fox and Solman's ZHF design in a thick aluminum housing with a cylindrical annulus construction that maintains radial temperature uniformity in the face of nonuniform radial heat flow.
The Fox/Solman and Togawa devices utilize heat flux normal to the body to control the operation of a heater that blocks heat flow from the skin through a thermal resistance in order to achieve a desired ZHF condition. This results in a construction that stacks the heater, thermal resistance, and thermal sensors of a ZHF temperature measurement device, which can result in a substantial vertical profile. The thermal mass added by Togawa's cover improves the tissue temperature uniformity of the Fox/Solman design and makes the measurement of deep tissue temperature more accurate. In this regard, since the goal is zero heat flux through the device, the more thermal resistance the better because it increases probe sensitivity. However, the additional thermal resistance adds mass and size, and also increases the time required to reach a stable temperature.
The size, mass, and cost of the Fox/Solman and Togawa devices do not promote disposability. Consequently, they must be sanitized after use, which exposes them to wear and tear and undetectable damage. The devices must also be stored for reuse. As a result, use of these devices raises the costs associated with zero-heat-flux DTT measurement and can pose a significant risk of cross contamination between patients. It is thus desirable to reduce the size and mass of a zero-heat-flux DTT measurement device, without compromising its performance, in order to promote disposability after a single use.
An inexpensive, disposable, zero-heat-flux DTT measurement device is described and claimed in the priority application and illustrated in FIGS. 3 and 4. The device is constituted of a flexible substrate and an electrical circuit disposed on a surface of the flexible substrate. The electrical circuit includes an essentially planar heater which is defined by an electrically conductive copper trace and which surrounds a zone of the surface that is not powered by the heater, a first thermal sensor disposed in the zone, a second thermal sensor disposed outside of the heater trace, a plurality of electrical pads disposed outside of the heater trace, and a plurality of conductive traces connecting the first and second thermal sensors and the heater trace with the plurality of electrical pads. Of course, the flexibility of the substrate permits the measurement device, including the heater, to conform to contours of the body where measurement is made. Sections of the flexible substrate are folded together to place the first and second thermal sensors in proximity to each other. A layer of insulation disposed between the sections separates the first and second thermal sensors. The device is oriented for operation so as to position the heater and the first thermal sensor on one side of the layer of insulation and the second thermal sensor on the other and in close proximity to an area of skin where a measurement is to be taken. As seen in FIG. 4, the layout of the electrical circuit on a surface of the flexible substrate provides a low-profile, zero-heat-flux DTT measurement device that is essentially planar, even when the sections are folded together. Of course, the flexibility of the substrate permits the measurement device, including the heater, to conform to contours of the body where measurement is made.
Operation of the heater of a zero-heat-flux DTT measurement device causes formation of an isothermal channel into tissue under the area of contact between the device and the skin of a subject. The zero-heat-flux DTT measurement is made by way of this isothermal channel. The larger the area of the heater, the larger the isothermal channel and the more deeply it penetrates into the tissue. The isothermal channel generally is at a higher temperature than the tissue which surrounds it, and so heat in the isothermal channel is lost to the surrounding tissue. This loss of heat reduces the size and depth of the isothermal channel.
Design and manufacturing choices made with respect to a zero-heat-flux DTT measurement device can influence the formation of an isothermal channel. Two such design choices relate to heater construction and measurement device size. In this regard, an important measure of heater performance is power density, the amount of power (in watts, for example) that a heater produces per unit of area (in square centimeters or cm2, for example). A convenient expression of power density is watts/cm2.
In a zero-heat-flux DTT measurement device, a heater with uniform power density does not generate a uniform temperature across its heat-emitting surface when the device is in contact with a semi-infinite solid, such as tissue. For example, if the circularly-shaped heater in the measurement device of FIG. 3 is invested with uniform power density in a radial direction, the temperature level drops along a radius of the heater in the direction of the periphery when the device is placed on skin. In other words, the heater is cooler near and up to its outer edge than near its center, and the isothermal channel through which core body temperature is measured is narrower than it would be if a uniform temperature were maintained in the radial direction. Consequently, presuming uniform power density, a progressively larger heater, and thus a larger measurement device, is needed to obtain reasonably accurate deep tissue readings when the measurement location moves from the forehead, to the neck, to the sternum. For example, a measurement device according to FIGS. 3 and 4 with a uniform density heater needs a first minimum diameter, for example about 30 mm (707 mm2), to accurately measure core body temperature at the forehead. However, such a uniform-power-density measurement device needs a second, larger, minimum diameter, for example about 40 mm (1257 mm2), for reasonable measurement accuracy on the neck. We have found that a uniform-power-density measurement device with a third minimum diameter, for example about 50 mm (1963 mm2), is too small to obtain reasonable accuracy through the sternum. We also note that Fox and Solman used a 60 mm square (3600 mm2) zero-heat-flux DTT measurement device with a uniform power density for measurement through the sternum.
However, a zero-heat-flux DTT measurement device fabricated in a single size with a uniform-power-density heater that meets performance requirements for the deepest core body temperature measurement might be too large to be used at other measurement sites. Depending on the location, space for taking a core body measurement can be limited, especially if other measurements are made nearby. For example, abdominal or thoracic surgery might require simultaneous measurement of brain activity, blood oxygen, and core body temperature. In such a case, an optimal measurement site for placement of BIS electrodes, an oxygen monitor, and a DTT measurement device would be on the patient's head; preferably the patient's forehead (including the temples) which is convenient to use, nonsterile, visible, and validated for measuring core body temperature. Manifestly, the forehead area available for placement of measurement devices can quickly become limited as the number of different measurements increases. Accordingly, constructions for a disposable, noninvasive, zero-heat-flux DTT measurement device should have a relatively small contact area. However, downward scaling of a uniform-power-density device can reduce the reliability of the temperature measurements produced by a smaller device for at least two reasons: deterioration of the isothermal channel through which DTT is measured and influence of nonpowered areas on temperature uniformity.
Generally, zero-heat-flux DTT measurement requires a heater with the capacity to deliver enough heat to create and maintain an isothermal channel to some required depth. Reduction of the size of the measurement device requires constructions that still deliver enough heat to create the isothermal channel and that do not compromise the uniformity with which the heat is delivered. However, as the size of the heater is reduced, the size and depth of the isothermal channel is reduced, making it more susceptible to being compromised by the effects of multidimensional heat loss in surrounding tissue. This effect can be more pronounced at measurement sites where the core temperature is relatively deep in the body, such as on the sternum.
Reduction of heater size can also increase the effect which nonpowered areas of the measurement device have on the temperature uniformity of the heater. For a measurement device fabricated by metal deposition techniques, the conductive traces for thermal sensors and other electronic elements deliver no heat and occupy areas which are not powered by the heater. In some designs, such unpowered areas penetrate the heater, thereby reducing the temperature uniformity of the measurement device.
Inconsistencies and irregularities in the thermal insulation near the first thermal sensor can influence its operation and cause it to produce faulty readings. As the size of the measurement device is reduced, these inconsistencies and irregularities increasingly compromise the uniformity of the temperature.
Finally, if additional electronic elements are added to a zero-heat-flux DTT measurement device, additional leads and connections must be provided, which increases the total nonpowered area of the device and additionally complicates the heater layout.